These are all the relationships Tobias has with other characters in the show.
Rachel is Tobias' older sister. They are very hostile to each other - Rachel screams at him and his friends that they aren't invited to her party, so Tobias has to blackmail her into inviting them.
Tobias and his mom seem pretty close. In "The Tape" after being embarrassed by his failed attempt at flirting with Masami, he calls his mom to pick him up early. In "The Mothers," Tobias says his mom is the best against Banana Joe's and Gumball's. In the mom-off that follows, he cooperates with his mom in cheating. However at the end she tears up her mother's day card for losing. Though she loses, they (the whole family) are seen waiting happily for Sluzzle Dude in "The Lie."
Rob hung out with Tobias throughout "The Party." Rob might be Tobias' good friend. However, now that Rob is the villain later on in the show, he and Tobias aren't friends anymore.
Tobias and Juke are friendly towards each other, and this is seen when they are shown many times playing basketball together. One instance was in "The Storm" when the ball they are playing with accidentally hits Gumball, and makes him kiss Carmen.
Idaho and Tobias have hanged out many times in episodes. They seem to be on good terms.
Like Gumball, Tobias has a crush on Penny. Unlike Gumball before "The Shell" when he was shy around Penny, Tobias was always more confident in his abilities to woo her, though Penny does not return Tobias' feelings for her because she only has them for Gumball. This is shown in "The Knights" where Tobias challenges Gumball for Penny's hand. Tobias attraction to Penny is evidenced again in "The Butterfly" in which he shows her his dance move "the chick magnet", she is already in her relationship with Gumball at this point and the way she puts it is that it goes really well with her dance move, which she shows him, telling him it's called "the bug spray".
In "The Test," Tobias is seems to make a move on Penny by asking her out on a dinner date. They also were holding hands together.
Practically Every Girl at Elmore Junior High
Throughout the events of "The Move" Tobias attempts to chat up numerous girls after the other, each of which rejects him for his repulsive arrogance and vanity. Beforehand to this, it is shown how much he views himself as a "chick magnet" even though no girl ever shows any sign of attraction to him, in episodes like "The Tape," in which he attempts to chat up Masami only to end up embarrassed.
Tobias seems to have an on-and-off friendship with Gumball. In "The Third", he becomes Darwin's and Gumball's friend for twenty dollars, but ignored Gumball and hung out with Darwin instead. When Gumball and Darwin reunite at Tobias' house. Tobias asks if he can play with them, which both of them deny. In "The Quest," he ignores Gumball's threat and tossed Anais' stuffed Daisy the Donkey out of the School Bus' window. In "The Knights," they "dueled" for Penny's hand.
Tobias was a good friend to Darwin in "The Third," but this may have been because he was paid twenty dollars to do so. He also followed Banana Joe in stopping Darwin kissing Masami, not to help him, knowing he didn't want to, but because he didn't want Gumball or Darwin to like girls just because he and Banana Joe didn't, in "The Pressure."
Tobias looks like a frenemy of Banana Joe, because sometimes they are seen together, and other times Tobias mocks him. In "The Pressure" they were the two Pals Before Gals members who kept up with their pact. Later in "The Club" they are both seen playing around in a friendly way in the football club. In "The Promise" It's mentioned that Tobias insulted Banana Joe's mom. In "The Mothers" They have a competition to see who is the best mother, and Tobias cheats, annoying Gumball, Darwin and Banana Joe.
Though they are not seen hanging out so much, in "The Move" Tobias reveals that Clayton had told him many things (most of which were lies). Before he finds out that Clayton has been lying, he seems to respect and even look up to Clayton, even though this is only based on his lies. Even after, like Gumball and Darwin, he seems friendly enough to Clayton until he knocks him out with the "finger-touching heart-disintegrating move."